The present invention relates to a device for applying a liquid, such as a photographic photosensitive liquid, a magnetic liquid or a surface protective liquid, to a flexible carrier such as a plastic film, paper or a metal leaf, hereinafter often referred to as web.
Conventional devices for applying a liquid to a web may be, for example, of the extrusion type, the curtain flow type, the blade doctor type, or the slide coat type. In these devices, the liquid is applied at a uniform thickness to the continuously moving web while the applied quantity of the liquid is regulated by the metal edge portion of an extrusion head, a slide coating head, or a blade, along which a narrow slot is defined. For example, an application head 1 of the extrusion type includes a pocket 3, a slot 4, a doctor edge portion 5 and a back edge portion 6, as shown in FIG. 1.
The pocket 3 is a liquid reservoir having a nearly circular cross section and extending with the cross section along the width of the web. The effective length of the pocket 3 along the width of the web is equal to or slightly larger than the width of the liquid-applied area of the web. Both the open ends of the pocket 3 extending through the body of the head 1 are closed by seal plates 7 fitted on both ends of the pocket 3.
The slot 4, which extends through the body of the head 1 from the pocket 3 toward the web, is a relatively narrow passage extending along the width of the web as well as the pocket. The length of the outlet of the slot 4 along the width of the web is nearly equal to the width of the liquid-applied area of the web. The doctor edge portion 5 and the back edge portion 6 are provided at the outlet edges of the slot 4 so that the doctor edge is located downstream to the back edge portion with regard to the direction of the movement of the web,
A liquid feed system (not shown in FIG. 1) is connected to a short pipe 9 projecting from the seal plate 7 so that a liquid C is filled into the pocket 3 and the slot 4 and then discharged from the slot with a uniform liquid pressure distribution along the width of the web.
The doctor edge portion 5 and the back edge portion 6 may be made of a very hard alloy, as disclosed in Japanese Unexamined Published Patent Applications Nos. 84771/82 and 104666/83, to increase the degree of rectangularity and flatness of the edge portions so as to enable the liquid to flow in a laminar manner from the pocket 3 with a uniform flow rate distribution and a uniform pressure distribution along the width of the web. Moreover, the wear resistance of the top of the application head 1 of the extrusion type, which is pushed toward the moving web in applying the liquid thereto, is enhanced.
The metallic doctor edge portion 5 is unavoidably eroded by the liquid discharged from the slot 4. This phenomenon is particularly conspicuous if the liquid contains an abrasive substance--for example, 20 parts or more by weight of the substance are added in the form of angular grains of 0.5 .mu.m or less in diameter to 100 parts by weight of a magnetic powder--as a magnetic liquid often does. Such wear is much caused not only in the application head of the extrusion type but also in the metal edge portions of the application devices of the curtain flow type, the blade doctor type, the slide coat type and so forth.
In recent years, the density of recording in a magnetic recording medium and the number of the various layers which form the medium have been increased. Along with such increases, the thickness of the layer of magnetic liquid applied to a nonmagnetic web in manufacturing the magnetic recording medium has had to be reduced. Also, it has been desired to increase the speed of application of the liquid to the web so as to raise the productivity of the process for producing the medium. It has also been desired to improve the magnetic properties of the recording medium by adding additives such as a lubricant and a dispersant to the magnetic dispersion liquid which is applied to a nonmagnetic web.
However, when such a magnetic liquid is applied at a high speed to a web by an application device of the extrusion type and whose application head is made of a very hard alloy, as disclosed in Japanese Unexamined Published Patent Applications Nos. 84771/82 and 104666/83, in such a manner that the thickness of the dried magnetic layer made from the applied liquid on the web is as small as 10 .mu.m or less, streaking tends to occur in the surface of the layer, namely, the liquid is not uniformly applied to the web.
Also, when a magnetic dispersion liquid containing an increased quantity of a lubricant is applied to a web by such a device, streaking tends to occur in the surface of the layer of the applied liquid on the web, namely, the liquid is not uniformly applied to the web. This is also a problem.
Carrying out repeated intensive studies, the present inventors have found that such streaking is due to the state of the surface of the very hard alloy of which the application head is made. Upon examining the surface of the doctor edge portion 5 of an application head made of a very hard alloy, it was found that the doctor edge portion had chipped areas 10 of about fifteen microns to scores of microns in mean diameter, as shown in FIG. 2. Such chips were found in almost all of edges of the application head. It is presumed that the chips result from the fact that the very hard alloy is composed of crystal grains of a carbide such as a tungsten carbide (WC), and a conjoining metal such as cobalt, which conjoins the crystal grains to each other.
When an application head is to be manufactured from a very hard alloy, the alloy is cut to a desired form and then accurately ground on the surfaces of the doctor edge portion and back edge portion of the head by a whetstone made of a material such as diamond. At that time, however, the crystal grains of the carbide, which are conjoined to each other by the metal, are not ground in part of the surface of the alloy, but instead are chipped from the surface so that hollows are left in the surface. Such hollows in the edges of the application head are the chipped portions In general, the greater the diameter of the crystal grains, the larger the size of the chips.
Examining the surface of an application head which causes streaking in the layer of the applied liquid on the web, the present inventors found the surface corroded. Such corrosion was found in almost all of such application heads made of very hard alloys. It is presumed that the corrosion results from the fact that the very hard alloy is composed of crystal grains of a carbide such as a tungsten carbide (WC), and a conjoining metal such as cobalt, which conjoins the crystal grains to each other. When the magnetic liquid is applied to the web by the application head, a higher fatty acid such as stearic acid, which is contained in the lubricant, dispersant or the like of the magnetic liquid, oxidizes the conjoining cobalt of the very hard alloy, which oxidization results in the corrosion. The oxidized surface of the application head gradually erodes so that the finished state of the surface deteriorates and the edges of the head become chipped or worn.
When a liquid is applied to a web by an application head having a doctor edge portion whose edges have such chips, the menisci of the liquid at the top of the application head are disturbed due to the chips or an extraneous substance contained in the liquid and then caught at the chips, causing streaking in the layer of the applied liquid on the web, and thus rendering it impossible to form a good layer of the applied liquid on the web. The adverse effects of the chips on the layer of the applied liquid on the web increase accordingly as the speed of the application is increased or the thickness of the layer is reduced.